[4.15] HUT OBSERVATIONS OF THE QUASAR E1821+64

G. Lee, G. A. Kriss, W. Zheng, A. F. Davidsen (JHU)

We obtained far-ultraviolet spectra of the quasar E1821+64 using the Hopkins
Ultraviolet Telescope during the flight of Astro-1 aboard the space Shuttle
Columbia in December 1990.
The HUT spectra cover the 912-1860 \AA\ wavelength range with a resolution of
$\sim$3 \AA.
In two successive orbits we accumulated
2966 s of high quality data during orbital night.
Our spectra extend well past the peak of the blue bump
down to a wavelength of 705 \AA\ in the rest frame of this
z=0.297 quasar.
There is no significant feature or change in spectral shape in the vicinity
of the Lyman edge. We model the spectrum with a relativistic accretion disk
with black body emissivities computed in the Schwarzschild metric. Our best
fit gives a central black hole mass of $\sim 13 \times 10^{8}~M_\odot$
accreting mass at a
rate of 19 $M_\odot~yr^{-1}$. This accretion disk spectrum cannot
account for the soft X-ray excess observed with ROSAT reported by Kolman et al.,
nor can Comptonized disk spectra simultaneously match the definite drop in UV
flux and the soft X-ray excess.

The strength of the soft X-ray excess depends sensitively on the
hydrogen column density along the line of sight.
From archival HST/FOS spectra we measure a
column density using the observed galactic damped Ly$\alpha$ profile of
$1.8 \times 10^{20}~cm^{-2}$.
This is significantly less than
the $4.1 \times 10^{20}~cm^{-2}$ suggested by the Stark et al. survey.
For a column density as low as $1.9 \times 10^{20}~cm^{-2}$, Kolman et al. find that
all the soft X-ray flux seen with ROSAT can be attributed to the central
star of the nearby planetary nebula K1-16.
We suggest that there is no significant
soft X-ray excess in E1821+64, consistent with the
relatively weak C\ IV and O\ VI emission in our spectra compared to Ly$\alpha$.